1. Field of the Invention
This invention relates to systems for reproducing color images by scanning an original and using electrical signals from the scanner to control the paint-spraying of a larger duplicate image. More particularly the invention relates to such a system in which a number of spray heads reciprocate across the imaging medium while the duration of pulses of ink or paint ejected by each spray head are controlled by pulse-width modulation of a constant pressure air stream that passes over a meniscus of the ink or paint.
2. Description of Related Art
Systems have been in use for making enlarged prints, such as for billboards, in which an original image is scanned to produce control signals that operate the reproduction equipment. In most systems, the recording medium is supported by a rotating cylinder while one or more spray heads slowly traverse the width of the cylinder and spray paint or ink in accordance with the scanning signals to reproduce the image on the medium carried by the cylinder. One such system is described in U.S. Pat. No. 1,709,926 in which the original and the recording medium are each carried by a rotating cylinder. A color duplicate of the original is produced by mechanically controlling three ink jets in accordance with signals produced by scanning the original while the ink jets are moved slowly across the width of the recording medium.
U.S. Pat. No. 1,817,098 describes a facsimile system using a first drum for scanning and a second drum for recording. The initial signal is divided into color components from which electrical control signals are generated. The control signals cause electric potentials to be applied to a pair of deflection electrodes positioned on opposite sides of a stream of atomized ink particles so that the ink stream is deflected toward the recording medium when an electrical potential is applied to the electrodes. When no voltage is present on the electrodes, the ink spray is prevented from reaching the recording medium.
U.S. Pat. No. 3,553,371 describes another dual-cylinder system in which multi-color images are reproduced by using one or more ink-jet heads operated in synchronism with the scanning signals. The pressure of the air is amplitude modulated in accordance with the scanner signals and the modulated air controls a mechanical valve that regulates the rate of ink flow. The extent to which the ink valve is opened is a function of the air pressure. The same patent also describes spray heads in which the ink jet is controlled by a mechanical valve operated directly by the scanner signals without pressure modulation of the air stream.
The above patent suggests the elimination of the recording cylinder by transferring the medium from one roll to another, forming an arcuate surface in the medium between the two rollers and paint spraying the medium by the use of a rotating head.
In most spray applications, it is preferred to use an internal system in which the ink and air are expelled together. Such an arrangement provides better atomizing of the spray, but it is not satisfactory where a specific pattern of color is to be reproduced requiring instantaneous response time in the control of the ink delivery.
Jets that include a mechanical ink valve operated by the recording signals are slow and suffer from problems associated with contamination, clogging and wear of the valve mechanism. Various attempts to amplitude modulate the air stream and avoid the problems associated with variable ink valves have not been commercially successful, in part because the character of the atomized ink is a function of the pressure of the air that produces the ink spray.